Conventionally, there has been studied a technique of improving the toughness of a construction in addition to its hardness and strength by coating the outer periphery of a continuous core material, such as fabrics, with other material. For example, U.S. Pat. No. 5,645,781 describes that a composite construction excellent in toughness indicating non-brittle fracture characteristic is obtainable by stretching by co-extrusion a laminated molding in which the outer periphery of a cylindrical core material molding composed of a first ceramic powder containing a large amount of organic binder (thermoplastic polymer) is surrounded by a shell layer molding consisting of a second ceramic powder different from the ceramic powder of the core material and an organic binder, and then sintering the stretched molding.
However, in the composite construction obtained by the method disclosed in U.S. Pat. No. 5,645,781, it is necessary to add a large amount of organic binder in order to perform co-extrusion molding. Therefore, during the time of sintering, a large amount of the organic binder are decomposed and volatilized, thereby forming voids. A large burning shrinkage occurs when these voids are eliminated for densifying the construction during the time of sintering. As the result, in the composite construction obtained by sintering, a large residual stress occurs between the core material and shell layer. In some cases, delamination between the two is facilitated and the strength of the composite construction decreases.
Further, with the above method, it is necessary to decompose and volatilize a large amount of the organic binder. In practice, binder burnout treatment has a limitation. Particularly, when there is a large amount of the organic binder remaining without being decomposed and volatilized, i.e., residual free carbon, in the core material located at the inside of the composite construction, the core material suffers from poor sintering and its sintered density does not increase, thus deteriorating the strength of the composite construction.